To accommodate people's busy lifestyles, portable electronic devices such as smart phones, tabular computers, and laptop computers have become indispensable. As an example, a smart phone equipped with a touch screen may not only include each feature provided by a traditional communication device, but may also serve as a mini personal computer that carries out versatile tasks such as document editing, e-mail serving, web serving, multimedia playing, photo shooting, and instant communication, and thus its usage has surpassed all the other electronic devices.
Since such portable electronic devices are still developed towards multifunctional applications to fulfill consumers' needs, wearable electronic devices such as a smart watch has become a continuously developing product nowadays. Despite of the wearability and mobile application that have been brought by the smart watch, its operation has been restricted due to its volume and screen size. Hence, to enhance its operational convenience under volume and size constraints is a problem to be solved by the developers.